1. Field of the Invention
This invention is related to electronic power amplifiers (PAs), and more particularly to radio frequency (RF) or saturated power amplifiers with multiple, selectable output power levels.
2. Related Art
Electronic amplifiers receive an input signal and provide an output signal that is typically a larger magnitude version of the input signal. As is well-known, class A, class AB, class B, or class C amplifiers are broad general categories of electronic amplifiers (there are also switching-mode amplifiers, such as class D, class E, class F2, etc.). The class A and AB amplifiers operate in a linear manner, while the class B and C amplifiers operate in a non-linear manner. The class A, AB, B, and C designations are generally determined by the position of the quiescent current point (Q-point) on the amplifier's load line, as set by a particular amplifier bias setting. The Q-point position is set by bias levels (e.g., voltages) applied to the electronic amplifier circuit components. These amplifier classifications apply to small signal inputs to wideband (untuned) amplifiers.
A tuned amplifier is an electronic amplifier that is configured to provide a maximum gain at a particular input signal frequency. For example, a tuned RF amplifier provides a maximum gain at a particular RF operating frequency or at a frequency within a particular RF operating frequency band (e.g., a cellular telephony (wireless) frequency band designated for operation by a government agency). The amplifier is tuned to provide maximum gain at the desired frequency by resonating capacitance with inductance in the amplifier circuit.
In an ideal amplifier, the output signal waveform exactly corresponds to the input signal waveform (i.e., a linear relationship), except that the output signal waveform is increased in amplitude and possibly time-delayed (phase shifted). All real world amplifiers, however, distort the input signal during amplification such that there is at least a small degree of non-linearity between the amplifier's output signal and the driving input signal. For example, distortion can occur because of the amplifier's inherent characteristics (e.g., non-linear signal response during operation) or because of extraneous signals affecting the output signal (e.g., noise).
In wireless (e.g., radio) communications systems, the signal carrying information being transmitted is typically a byproduct of a carrier signal component and a modulating signal component. The modulating signal carries the transmitted information and is used to alter (modulate) the carrier signal waveform. Various well-known modulation techniques include amplitude modulation (AM), frequency modulation (FM), pulse code modulation (PCM), Gaussian minimum shift keying (GMSK), and coded modulation schemes (e.g., coded orthogonal frequency division multiplexing (COFDM)). In wireless communication systems, both carrier signal and modulating signal distortion are important design factors. Since the modulating signal component of the signal being amplified carries the information, modulating signal distortion should be minimized.
Three types of distortion that are of interest in RF amplifiers are harmonic distortion of the input signal to the amplifier, amplitude distortion of the modulating signal component of the input signal (e.g., AM-AM envelope distortion), and phase distortion of the modulated signal due to input envelope changes (e.g., AM-PM distortion). Since amplifiers used in RF applications are typically tuned amplifiers, harmonic components of the input signal are suppressed to acceptable levels in the amplifier's output signal. But RF amplifiers must amplify the input signal such that the modulating component in the amplified output signal is acceptably linear (acceptably distortion-free). If not driven into saturation by the input signal, both class A and class AB tuned RF amplifiers typically provide the required modulating signal component linearity for modern wireless power amplification applications (e.g., amplifying cellular telephone signals for transmission by the handset).
A saturated amplifier is an amplifier (regardless of amplifier class rating) that is operated with high input signal overdrive such that the amplifier is severely voltage limited. That is, in a saturated amplifier, the output signal voltage is limited by the supply voltages applied to the amplifier (i.e., the top-rail and bottom-rail voltages). Therefore, the amplified output voltage signal waveform is clipped. Both the output voltage and current are moving through the available extremes of the amplifying component(s) (e.g., in both the ohmic/high current regions and cutoff/high voltage region). A saturated amplifier is a non-linear amplifier (i.e., the output signal waveform is a clipped version of the input signal waveform). However, when using some modulation schemes where the envelope of the input signal is constant, the amplitude-limiting characteristic of a saturated amplifier is acceptable. Modulation methods of this type typically use phase or frequency variations to carry the modulated information signal. Low AM-PM distortion is important. Saturated amplifiers advantageously provide high DC operating power-to-RF output power efficiency with low AM-PM distortion.
The Doherty amplifier has been known since the 1930s, and was first applied to radio signal broadcasting. The Doherty amplifier includes a carrier amplifier and a peaking amplifier, which operate in response to a variable amplitude input signal. When the envelope peaks of the input signal are less than an input threshold, the carrier amplifier operates in a non-saturated linear manner and provides low power amplification. The peaking amplifier is disabled at this time. When the envelope peaks of the input signal are greater than the input threshold, the carrier amplifier operates in a saturated mode, and the peaking amplifier is enabled. The peaking amplifier is biased for class B and/or class C bias operation. The peaking amplifier never saturates in the classic Doherty amplifier.
Electric power available to portable wireless (e.g., radio) transceivers (e.g., cellular telephone handsets) is typically limited by battery life and desired talk-time. Of the electronic components in such transceivers, the power amplifier in the transmitter section typically consumes the most electric power. As such transceivers are increasingly miniaturized, electric power consumption becomes a critical design consideration, because long battery life is desirable. Some transceivers have transmitter sections that output signals at selectable power levels. For example, in a typical cellular telephone handset operating under the Global System for Mobile Communications (GSM), the power of the RF input signal provided to the final stage amplifier of the transmitter is high and relatively constant. The output power of the final stage amplifier is controlled with an analog control signal. The output power of the final stage amplifier is reduced as the analog control signal is reduced, but since the battery voltage and power amplifier load impedance are constant, the DC to RF power efficiency is also reduced.
It is therefore desirable to have a saturated amplifier circuit that provides selectable output power levels and exhibits a high power efficiency over the desired range of power levels.